Materials Development and Architecture Engineering for High Efficiency Organic Solar Cells

• Author / Creator

  Cao, Bing

• The growing population and world economy has led to the rise in both the demand for energy and its production. Use of fossil fuels and abusive mining contribute to environmental pollution and the depletion of natural resources. In order to meet future energy demands and control CO2 emissions, low-carbon, renewable energy sources must be developed. Solar energy is of particular interest since just one- hour of sunlight could power the entire world for a year. An attractive method for harvesting solar energy is the use of photovoltaic devices, which directly convert solar energy into electricity. Organic semiconductor solar cells have emerged as a promising candidate because of their low-cost, lightweight, freedom in shape, ease of processing, and low environmental impact. The focus of this dissertation is to investigate mechanisms limiting polymer solar cell performance and ways to improve device efficiency. These approaches include developing photoactive and interfacial materials, as well as interface and device engineering. First, two isostructural low-band-gap conjugated small molecules were synthesized with one-atom substitution, S, and Se for the use as photon-absorbing material in organic solar cells (OSCs). The two molecules were based on the benzo[1,2-b:4,5-b′]dithiophene electron-rich unit and the sulfur-containing electron-deficient benzothiadiazole (BT) unit following the donor-acceptor concept. The investigation into the photovoltaic properties showed that one-atom substitution could engender substantial differences in the solubility, which then influenced the crystal orientations of the small molecules within this thin layer, resulting a higher power conversion efficiency (2.6%) for the Se-containing molecules. Second, we introduced interfacial layers to improve the OSC performance. A thin, low-band- gap polymer interfacial layer called PBDTTPD-COOH was synthesized and applied on the commonly used ITO/PEDOT:PSS electrode to modify the interface between ITO/PEDOT:PSS and bulk heterojunction (BHJ). The use of this modifier layer improved the polymer conversion efficiency for the PCDTBT- and PBDTTPD- based solar cell from 6.2% to 7.0% and from 6.3% to 7.3%, respectively, but had little effect on the PTB7-based solar cells. The investigation on the energy level alignment, phase segregation, and the local composition of the BHJ suggested that phase separation near the interface caused by the surface energy change was the key to efficiency improvement. A guideline for matching an interfacial layer and a BHJ with regard to the energy level pinning and phase segregation was proposed. Finally, we carried out an overall device engineering in novel structure to improve the charge extraction in solar cell devices. A nanostructured ITO electrode was deposited and used in a high-hole-mobility polymer:PCBM solar cell to achieve balanced charge extraction and collection. The ITO nanotree electrodes with a range of heights were fabricated into solar cells, and photovoltaic performance was characterized. The optimized efficiency was obtained at ITO nanoelectrode height of 75 nm. Investigation on the effect of the ITO indicated that shortening the electron extraction pathway, altering the potential distribution throughout the BHJ, and trapping light for photon harvesting contributed simultaneously to affect the photovoltaic performance.

• Subjects / Keywords

  • Bulk Heterojunction
  • Organic Solar Cells
  • Conjugated Small Molecules
  • Conjugated Polymers
  • Organic Photovoltaics

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3XK8543F

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.